1. Field of the Invention
The present invention relates to an improvement of a control device for a hydraulic actuator in a piston, in which one end of an oil passage that is provided through a connecting rod, a crankshaft and a crankcase supporting the crankshaft is connected to a hydraulic chamber of a hydraulic actuator provided in a piston connected to the crankshaft via the connecting rod with the other end of the oil passage is connected to an oil reservoir and a hydraulic pressure source via a main switching valve. The main switching valve moves between a first switching position which allows the oil passage to communicate with the oil reservoir, and a second switching position which allows the hydraulic pressure source to communicate with the oil passage.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2005-54619 discloses a control device for a hydraulic actuator in a piston.
In the conventional control device for a hydraulic actuator in a piston, the hydraulic actuator sometimes does not return to a non-operating state although the switching valve which should return the hydraulic actuator in an operating state to the non-operating state is switched to the first switching position thereby allowing the oil passage to communicate with the oil reservoir. The inventors of the present invention have found out that this occurs due to the following cause.
Namely, even when the switching valve is switched to the first switching position to allow the oil passage to communicate with the oil reservoir, operating oil remains in the oil passage in the connecting rod. The residual oil has an upward inertia force due to the mass of the residual oil itself when the connecting rod and the piston move downwardly, and the inertia force acts on the hydraulic chamber of the hydraulic actuator as a pressure. When the connecting rod and the piston move upwardly, the residual oil has a downward inertia force, so that the pressure of the hydraulic chamber of the hydraulic actuator is reduced. However, this time period of the reduced pressurize is too short for the hydraulic actuator to return to the non-operating state. In addition, the pressure by the upward inertia force becomes larger as the engine rotational speed becomes higher, and the pressure reduction time period of the hydraulic chamber of the hydraulic actuator becomes short. Therefore, the hydraulic actuator is difficult to return to the non-operating state especially during a high-speed rotation of the engine.